This invention relates to a twin-belt continuous casting mold, particularly for casting steel. The mold chamber is formed of parallel length portions of cooled upper and lower casting belts as well as parallel length portions of endless side dams which bound the mold chamber laterally. In the zone of the mold chamber the side dams are backed by adjustable guide rails. The continuous caster has upper and lower caster frames which respectively support the end drums for the upper and lower casting belts as well as a series of support rollers for backing up the casting belts. There are further provided seals which are in engagement with the casting belts and which serve for sealing off the mold chamber from the environment.
In known twin-belt continuous casting molds of the above-outlined type, such as disclosed, for example, in German Pat. Nos. 1,268,319 and 1,433,036, the width of the casting belts is so designed that they project, together with the associated support rollers, significantly beyond the side dams. Unless, as an exception, ribbon-like castings with significant width are manufactured, the width of the casting belts is a multiple of the width of the mold chamber: in case of a casting width in the order of magnitude up to 200 mm, the casting belts have a width which is approximately three times that of the mold chamber.
The space adjacent the side dams is needed for accommodating the guide rails supporting the side dams as well as the adjusting devices therefor and further, for a secure sealing of the mold chamber to protect it against the large quantities of coolant.
The prior art constructions have the disadvantage that the casting belts will become hot only in the zone of the mold chamber--that is, along a central longitudinal belt area--whereas in the zone adjacent the side dams they have the lower temperature of the admitted coolant. The non-uniform temperature distribution over the width of the casting belts is particularly disadvantageous if materials having a very high melting point--such as steel--are being processed. Tests have shown that the casting belts in the zone of the mold chamber have a mean temperature of approximately 112.degree. C. as compared to merely 20.degree. C. at the outer zones.
The above-noted temperature differences cause a non-uniform heat expansion of the casting belts in the longitudinal and transverse direction and lead to deformations of the casting belts. Such deformations cannot be entirely compensated for even if large forces are applied, because the non-expanded, cold outer zones of the casting belts cannot be stretched beyond a predetermined extent, due to the involved additional stresses. Particularly the transitional zones between the hot central parts and the cold outer parts of the casting belts are endangered.
Apart from the fact that the service life of the casting belts is significantly reduced because of the unfavorable temperature distribution, a non-uniform expansion of the casting belts has the following disadvantages:
Between the casting belts and the side dams gaps appear which lead to sealing failures in the zone of the mold chamber. Further, in the zone of the gap tongue-like deposits (fins) appear which grow in the casting direction and lead to damages of the side dams at the mold chamber outlet. An expansion of the casting belts in the longitudinal direction is obstructed by the cold outer zones; this condition leads to the appearance of gaps between the casting and the casting belt since the latter is held only by point-like contacts on the support rollers. The air gaps adversely affect, by virtue of their heat insulating properties, the efficiency of the coolant of the twin-belt continuous casting mold so that the mold chamber has to be relatively long in order to obtain a self-supporting casting skin. It has been attempted to limit the size of the air gap by an outwardly convex design of at least those casting belt support rollers which are immediately upstream of the mold chamber outlet. Such rollers thus have a varying diameter which has its maximum situated in the middle of the roller length.
As a result of the non-uniform temperature distribution, the cold flanking zones of the casting belts are stressed taut while the hot, significantly expanded mid zones deform in an indeterminate direction. Thus, the casting belt may either bulge towards the casting whereby a satisfactory heat contact is obtained or it may bulge away from the casting, resulting in an air gap, causing a poor heat contact. Apart from the fact that the direction of bulging of the casting belt cannot be foreseen, the deformation during the casting process cannot be influenced, because at any moment the bulged deformation of the casting belt may snap into an oppositely oriented configuration. Thus, zones of good heat contact between casting and casting belt are suddenly transformed into zones of poor heat contact. The inferior cooling effect of the twin-belt continuous casting mold involved with such phenomena may cause, in the worst case, a complete rupture of the casting, leading to an interruption of the casting operation.